Synchronous satellites orbit the Earth with the same revolution rate as that of the Earth. Accordingly, the satellite appears above a fixed point on the Earth. Hence, synchronous satellites are often referred to as geostationary satellites and operate within a stationary orbit. Synchronous satellites are useful for many applications including weather and communication applications.
It is generally well known in the art that various forces act on synchronous satellites to move the satellite out of stationary orbit. These forces are due to several sources including the gravitational effects of the sun and moon, the elliptical shape of the Earth and solar radiation pressure. To counter these forces, synchronous satellites are equipped with propulsion systems that are fired at intervals in order to maintain station in a desired orbit. This requires control of the inclination, eccentricity and mean motion of the satellite. Inclination is the north-south position of the satellite relative to the Earth's equator. Eccentricity is the measure of the non-circularity of the satellite orbit. That is, the measure of the variation of the distance the satellite is from the Earth as the Earth and satellite rotate. Finally, mean motion is the average position of the satellite in an east-west direction relative to a sub-satellite point on the Earth. For a more detailed discussion see Controlling a Stationary Orbit Using Electric Propulsion by Bernard M. Anzel, presented to the 1988 International Electric Propulsion Conference in West Germany.
Station keeping was first achieved with a spin-stabilized communication satellite launched in 1964. Current satellites are either spin-stabilized or three-axis stabilized satellites. Spin-stabilized satellites use the gyroscopic effect of the satellite spinning to help maintain the satellite orbit. For certain applications, the size of the satellite militates in favor of a three-axis stabilization scheme. Current three-axis stabilized satellites use separate sets of thrusters to control north-south and east-west motions. The north thrusters produce the required north-south change in satellite velocity, or .DELTA.V, to control orbit inclination. The east thrusters and west thrusters produce the required combined east-west .DELTA.V to control orbit mean motion and eccentricity. For each of these three maneuvers, thrusters are fired in pairs to cancel torques since the thrust directions do not pass through the satellite center of mass. Furthermore, there are three separate maneuvers performed at different times as required by the physics of the perturbations. The frequency of these maneuvers are typically every 14 days for both the north-south maneuver and the pair of east-west maneuvers (east and west firings occur approximately 1/2 orbit apart or about 12 hours) when using 5 pound thrusters with liquid propulsion.
In U.S. Pat. 5,020,746, assigned to the assignee of the present invention and incorporated herein by reference, station keeping of a three-axis stabilized satellite is provided using only two thrusters mounted on the anti-nadir face of the satellite. A north thruster is canted away from the face at an angle .theta. from the north-south axis of the satellite in a northern direction and a south thruster is canted away from the face at an angle .theta. from the north-south axis in a southern direction. Both thrusters are also translated to the east or west along an east-west axis of the satellite and swiveled at variable angles .alpha..sub.1 and .alpha..sub.2, respectively. The patent discloses a technique for determining the angles .alpha..sub.1 and .alpha..sub.2 and the firing positions of the thrusters in order to maintain the satellite in a stationary orbit.
A two thruster system such as disclosed in the aforementioned patent is constrained in several respects. In order to control east-west motion as well as north-south motion, the thruster mounting locations must be customized for the particular satellite station location. Notwithstanding this customization only a partial control of eccentricity is achieved. Also, in order to counter the eccentricity buildup resulting from a failure of one of the two thrusters, a back-up thruster must be provided.